As is known in the art, Diesel Exhaust Gas Recirculation (EGR) coolers provide a cooled dilutent to lower combustion temperatures and reduce the concentrations of Oxides of Nitrogen in the exhaust gases. Such coolers typically include shell and tube heat exchangers using engine coolant. Due to the size of the EGR coolers, they must be packaged horizontally to fit into the engine compartment.
As is also known, diesel exhaust contains particulates and hydrocarbons, which together can adhere to the walls of the heat exchanger tubes of the cooler and thereby reduce the heat transfer performance of the cooler. This adhering behavior is made worse with lower gas temperatures leading to condensation of hydrocarbons and other exhaust constituents on the walls of the coolers which thereby collect more particulates. Due to new combustion and aftertreatment strategies to meet US Tier 2 emissions standards in diesel engines, there will also be even higher concentrations of Hydrocarbons with higher condensation temperatures that will worsen the adhering behavior. Researchers have proposed several basic forces which drive the contaminants to the walls including: 1) the temperature gradient across the tubes; 2) the pressure gradient created during condensation; 3) electrostatic forces; 4) inertial forces; 5) diffusion; and 6) gravity. The introduction of turbulence inducing ribs to improve heat transfer performance can make the fouling effect worse, making it desirable to further reduce the effects of hydrocarbon condensation and particulate deposition.
Another issue with current EGR subsystems is the packaging of sufficient ‘mixing’ length downstream from the point where the EGR gases enter the intake manifold. With the very high percentages of EGR required for future emissions standards, mixing lengths of 400 mm and longer may be required to achieve a homogeneous mix of EGR and fresh air. These long mixing lengths are difficult if not impossible to package and drive intake manifold designs that are disadvantageous for other important attributes such as volumetric efficiency, runner-to-runner flow balance, packaging, cost and weight.